1. Field of the Invention
The present invention relates to an inspection method of a sensor device using a capacitive sensor element, and in particular, to an inspection method of a sensor device including a circuit for converting the capacitance of a capacitive sensor element into a voltage.
2. Description of the Related Art
As electric humidity sensors, a resistive humidity sensor using a sensor element having an electrical resistance that changes according to humidity and a capacitive humidity sensor using a sensor element having a capacitance that changes are generally known.
FIGS. 14A and 14B are diagrams showing the configuration of a resistive humidity sensor. A sensor unit 100 is a circuit including a sensor element having a resistance that changes according to humidity, and a DC voltage is applied to the sensor unit 100. When the resistance of the sensor element changes, the voltage of the signal line also changes according to the resistance change. An amplifier circuit 101 amplifies the voltage of the signal line, and outputs the amplified voltage as a humidity detection signal.
On the other hand, FIG. 15 is a diagram showing the configuration of a capacitive humidity sensor. A sensor unit 110 is a circuit including a sensor element having a capacitance that changes according to humidity, and an AC voltage is applied to both ends of the sensor unit 110. When the capacitance of the sensor element changes, electric charges according to the change are accumulated in a capacitor 114. An operational amplifier 113 outputs a voltage corresponding to the electric charges accumulated in the capacitor 114 as a humidity detection signal.
In the case of the resistive sensor using a sensor element having an electrical resistance that changes, as shown in FIGS. 14A and 14B, it is possible to make a constant current flow through the signal line using a constant current source 102. If the signal line is broken, the voltage of the signal line is reduced to the ground level by the current of the constant current source 102. Accordingly, the output voltage of the amplifier circuit 101 becomes an abnormal value. Thus, in the resistive humidity sensor, it is possible to inspect the breakage of the signal line relatively easily using the constant current source. However, in the case of the capacitive sensor using a sensor element having a capacitance that changes, as shown in FIG. 15, a circuit configuration for inputting electric charges corresponding to the capacitance to charge amplifiers 113, 114, and 115 and converting the electric charges into a voltage is common. When a constant current source is connected to the signal line for transferring the electric charges to the charge amplifier from the sensor unit, the charge amplifier integrates the current of the constant current source. Accordingly, electric charges corresponding to the capacitance of the sensor element cannot be detected successfully. That is, the capacitive sensor has a problem that the breakage of the signal line cannot be detected using the constant current source unlike the resistive sensor.
In addition, in the case of the capacitive sensor, as shown in FIG. 15, an AC voltage is applied to the sensor unit 110. For this reason, the sensor unit 110 and a driving circuit of AC voltage need to be connected to each other by wire bonding or the like. In this case, the wiring line may also be broken. When performing wire bonding, the wires may be short-circuited. Therefore, in the capacitive sensor, not only may the signal line be broken, but also the wiring lines of the driving circuit may be broken or may be short-circuited. It has been an issue to inspect these failures appropriately.